Is this thread safe?

[Walter Bright]

dsimcha wrote:
> I'm very new to concurrent programming and am trying to implement some high
> performance math stuff in D.  Could someone please tell me whether the
> following simple function would be considered thread safe?

No, it is not thread safe. It's the old double checked locking problem. 
The reasons are difficult to understand, but understanding it is crucial 
to writing thread safe code.

http://www.ddj.com/184405726

http://en.wikipedia.org/wiki/Double_checked_locking_pattern